1. Field of the Invention
The present invention generally relates to a drive-voltage control device and, more particularly, to a drive-voltage control device that controls a drive voltage supplied to an apparatus having a power-saving power-supply circuit. The present invention also relates to an image forming apparatus having the drive-voltage control device.
Recently, many image forming apparatuses such as a copy machine, a printer or a facsimile apparatus are provided with a power-saving power-supply circuit that outputs a plurality of power-supply voltages. The power-saving power-supply circuit outputs a control voltage and a drive voltage when a power-saving mode is turned off, and outputs a waiting voltage instead of the control voltage and the drive voltage when the power-saving mode is turned on.
A liquid crystal display device provided in such an image forming apparatus is one of the devices to which a plurality of power supply voltages are supplied. The liquid crystal display device generates a timing signal in accordance with the control voltage supplied by the power-saving circuit. The liquid crystal display device provides the drive voltage supplied by the power-saving power-supply circuit or a voltage generated based on the drive voltage to electrodes of the liquid crystal display device in accordance with data corresponding to an image to be displayed on a liquid crystal display.
2. Description of the Related Art
In recent years, an international rule has been suggested which rule is related to a power-saving function of image forming apparatuses when they are set in a waiting state. According to such a power saving function, an unnecessary power-supply voltage is cut off in the power-saving mode. Conventionally, the power-saving mode is achieved, on one hand, by continuously providing a waiting voltage to a monitoring unit such as a CPU and a memory unit, and, on the other hand, by interrupting a drive voltage provided to a circuit consuming a large power such as a motor driver, a solenoid driver or an image developing circuit during a waiting time and providing the drive voltage when an instruction is provided to do so. Japanese Laid-Open Patent Application No.8-69225 discloses an image forming apparatus which supplies a reduced voltage to a liquid crystal display device and displays a minimum image on the liquid crystal display device during a waiting time.
In any case, a power-saving power-supply circuit outputting a plurality of power-supply voltages is used which power-saving power-supply circuit outputs both the control voltage and the drive voltage when the power-saving mode is turned off and outputs the waiting voltage instead of the control voltage and the drive voltage when the power-saving mode is turned on.
FIG. 1 is a block diagram of a conventional power-saving power-supply circuit. When a power switch 1 is turned on, an AC voltage provided through a plug is supplied to a primary rectifying circuit 2. The AC voltage is rectified into a DC voltage by the primary rectifying circuit 2. The DC voltage output from the rectifying circuit 2 is provided to three DC/AC converters 3a, 3b and 3c. Each of the DC/AC converters 3a, 3b and 3c converts the DC voltage into an AC voltage so as to decrease the AC voltage. The decreased AC voltage output from each of the DC/AC converters 3a, 3b and 3c is provided to a respective one of secondary rectifying circuits 4a, 4b and 4c so that different direct current (DC) power supply voltages are output from the secondary rectifying circuits 4a, 4b and 4c. In FIG. 1, +5 V is output from the secondary rectifying circuit 4a; +12 V is output from the secondary rectifying circuit 4b; and +24 V is output from the secondary rectifying circuit 4c. A supply line of the +5 V is branched into two lines, and a switch 5 is provided to one of the branched lines. The +5 V on the other one of the two branched lines corresponds to a waiting voltage provided to circuits which must be continuously operated even when the power-saving mode is turned on. This voltage is referred to as +5VE as shown in FIG. 1.
When a power-saving mode is turned on, the DC/AC converters 3b and 3c are turned off in accordance with a signal sent from a control unit of an image forming apparatus (not shown in the figure). Additionally, when the power-saving mode is turned on, the +5V is interrupted by turning off the switch 5. Accordingly, when the power-saving mode is turned on, the +5 V, the +12 V and the +24 V are interrupted and only the +5 VE is output from the power-saving power-supply circuit.
FIG. 2 shows a time chart of the DC voltages output from the power-saving power-supply circuit shown in FIG. 1 when the power switch 1 is turned on and off. In FIG. 2, voltages at the +5 V line and the +24 V line are indicated. As shown in FIG. 2, each of the voltages at the +5 V line and +24 V line is gradually increased after the power switch 1 is turned on, and is gradually decreased after the power switch 1 is turned off. There is not a large difference between a time for the voltage at the +24V line to become a constant voltage and a time for the voltage at the +5 V line to become a constant voltage. The time needed for each of the voltages at the +5 V line and the +24 V line to become a constant voltage is dependent on an operation of the DC/AC converters 3a and 3c and a state of a load connected to the +5 V line and the +24 V line.
FIG. 3 shows a time chart of the voltages at the +5 V line and the +24 V line when the power-saving mode is turned off, and, thereafter, turned on. When the power-saving mode is turned off and on, the voltage at the +24 V line is interrupted in the same manner as when the power switch 1 is turned on and off. On the other hand, the voltage on the +5 V line is controlled by turning on and off the switch 5. Accordingly, a rate of increase and decrease in the voltage at the +5 V line is much larger than that of the voltage at the +24 V line. Particularly, a rate of decrease in the voltage at the +5 V line is much larger than that of the voltage at the +24 V line.
The control voltage +5 V is mainly used for control circuits. Some parts in the image forming apparatus to which the power-saving power-supply circuit shown in FIG. 1 is connected, are supplied with the control voltage +5 V for a controlling circuit and the drive voltage +24 V for a driver circuit. In these parts, if the voltage for the controlling circuit is interrupted before the drive voltage is interrupted, the parts or the driver circuit in the parts may be damaged. A liquid crystal display device provided in an operational panel of the image forming apparatus is one of the parts which may be damaged when the control voltage is interrupted before the drive voltage is interrupted. If the liquid crystal display device is damaged, a user is negatively influenced. Thus, such a problem must be eliminated.